In one type of conventional outboard motor, a propeller is driven by a powerhead to propel a boat through water. Most large outboard motors of this type inject the exhaust gas stream under water in order to reduce engine noise.
In a typical configuration shown in FIG. 1, the gas exhausted from the powerhead 10 flows downwardly through an exhaust channel 12 and exits the motor rearwardly through the propeller 14. This type of motor is referred to as an exhaust-through-hub (ETH) motor.
A boat powered by an outboard motor equipped with a conventional open propellor generates a unique noise signature which is characterized by a three-dimensional noise field similar in shape to an ice cream cone lying on its side. The size of the noise field is determined by the propeller size, the number of propellor blades, the rotational speed of the engine. etc.
For many applications, the capability to restrict the size of the emitted noise field, as well as change its spectral intensity over the field of interest (e.g., audible near-field alteration), is desirable. A shroud or propeller guard can be used to achieve part of the noise field reduction, but installation of a shroud or propeller guard is accompanied by a loss in performance of the propulsion system.
Another type of conventional outboard motor has an axial-flow pump jet system driven by the powerhead. In a pump jet system, an impeller or rotor is mounted (e.g., spline fitted) directly on the propeller output shaft in place of the propeller. There are typically no modifications to the drive train, cooling or sealing components. A ducted housing surrounds the rotor. Such a system has the advantages of reducing hazards to swimmers in the vicinity of the motor, protecting the rotating elements from interference with and damage by foreign objects in the water, and improving the efficiency and performance of the propulsion system. Another benefit inherent with the pump jet is a directed jet of water that results in greater steering response. More pertinent to the present invention, the pump jet propulsor, with its high-efficiency shrouded design, alters the radial noise field and provides an opportunity to also alter the noise spectrum.